Aqueous polyethylene dispersions produced by subjecting molten polymer in water to ahigh rate of shear



Patented Sept. 29, 1953 AQUEOUS POLY-ETHYLENE DISPERSIONS- PRODUCED BY.SUBJECTING MOLTEN POLYMER; IN WATER TO A HIGH RATE OF SHEAR EdwardHunter, Northwich, England, assignor to Imperial Chemical IndustriesLimited, a corporation of 'Grea-tBritain No Drawing.

Application September 6, 1950,

Serial No. 183,475.. In Great Britain September 28, 1949- 10 Claims. 1,

Several. methods have been described. for the manufacture of dispersionsof polythene referred to hereinafter as normally solid polyethylene.They include aqueous polymerisation to .give polymers directly as anemulsion; dissolving the material in. a solvent, emulsifying thesolution with Water, and removing the solvent; and milling the solidmaterialwith dispersing. agents or protective colloids, and addingwater. These methods rarely give good quality stable dispersions, andare often expensive.

We have found that although normally solid. polyethylene does notproduce a free-flowing liquid when melted, and. has a-high viscosity,yet we can emulsify molten polythene with water ina colloid mill orsimilar device.

According to the presentiinvention, we manufacture dispersions ofnormally solid polyethylene by a process which, comprises passingmoltennormally solid polyethylene with water, at a temperature above 115C. and at sufficient pressure to keep the water in "the liquid'state,through an emulsifying device, and then releasin'g'the pressure.

Convenient emulsifying devices are any machines which provide highrates; of shear and keep a substantial pressure on' the materials besurecasing. The rate of shear is one of the factors which determines theparticle size of the dispersed norm'allysolid polyethylene, but asuitable rate is provided by either of these types of emulsifyingdevice. Colloid mills have two surfaces situated within. aboutone-hundredth of a centimetre moving at high speed'relative'to eachother, and the rate of shear is-determined by the speed and the gap.They canbe adapted so that the mixture is under pressure. Suitableconditions for a 10 centimetre diameter mill, for example, are that thegap shall be between and thousandths of a centimetre. and the rotorshall rotate atbe'twe'en 5000 and" 15,000 revolutions per minute. Whenworkin inza homogenizer we pump, hot liquidwat'er under pressure, withmolten polythene, through a narrow opening between a valve and its seat;the rate of shear is determined by the rate of flow and the size of gap.Both machines are modified so that the water feed and the polythene feedare hot and above two atmospheres pressure.

The temperature of operation must be above 115 (3., which is the meltingpoint of normally solid polyethylene, and as normally solid polyethylenedoes not produce a free-flowing liquid 2 when melted, it is preferablyat least 130 The'top temperature is limited both by the criticaltemperature above which water cannot be a liquid and bythe'decomposition of normallysolid polyethylene, and preferably does notexceed'2 '50 0;; when emulsifying agents are present it is limited bythe temperature at'which'they become inactive, which is generally about160 C. The minimum pressure needed exceeds the vapour pressure of waterat the operating temperature, and is thereforeat least2 atmospheresabsolute and generally at least 3-6' atmospheres. For convenience,however, we prefer to use rather high pressures between 20 andatmospheres.

The amount of water used is between 1 and 5 parts by weight per part ofnormally solid polyethylene, preferably between 2 and 5 parts by weightper part of normally solid polyethylene. It. is not commerciallyattractive to make dispersions which contain as little as 15% ofnormally solid polyethylene, and technical difiiculties arise in makingdispersions containing as much as 35% of solid ethylene polymers. Wealso prefer to add an emulsifying agent inorder to assist in preparingthe dispersion and to increase its stability. Agents which are notdecomposed at 'the'emulsiflcation temperature are effective, and ofthese we prefer the fatty acid compounds known broadly as soaps, forexample the potassium and sodium salts of stearic, palmitic, and otherlong chain carboxylic acids, as-

well as compounds of these acids with other bases such astriethan'ol'am'ine. "They are most advantageously introduced byincorporatingthe carboxylic acid in the normally solid polyethyleneand-dissolving the basic compound in the water; so "that duringemulsifl'cation the agent is formed in 'situ. Theamount of agentrequired is usually between 1 and 20 parts by weight per parts ofnormally solid polyethyl'ene, the optimum result beingobtained'withbetween 5 and 15 parts.

The water and normally solidip'ol'yethylenecan within about 10 secondsof leaving the emulsifying device, in order to avoid coalescence of theparticles. This is done by making the connecting space between theblow-off valve where the pressure is released and the outlet of theemulsifying device as small as possible. The releasing of the pressurecan be done into the air so as to allow steam to flash oil and thus coolthe emulsion to 100 C., but we prefer to cool further than this byblowing off the dispersion direct into some cooled dispersion at 3060The normally solid polythene particles in the dispersions obtained bythis process can readily be made as small as 5-20 microns.

The invention is illustrated by the following example.

Example A colloid mill with a gap of .01 cm. and a cm. diameter rotor ina steel casing to withstand pressures up to 100 atm. was fitted with apressure release valve for delivering the dispersion, a pressure tightgland for the drive, and a steam jacket. It was also fitted with twofeed pipes, one of which led from a water heater fed by a pump from astock tank while the other led from a heater for the normally solid polyethylene fed by a screw extrusion pump. 300 kgm. of water with 6.6 kgm.of triethanolamine were mixed in the stock tank. 100 kgm. of normallysolid polythene (Alkathene brand of polythene, Grade 70, Alkathene beinga registered trade-mark) were mixed with 10 kgm. of stearic acid onsteam-heated rubber milling rolls and fed into the hopper of the screwextrusion pump. The aqueous phase was fed into the mill at a rate of 20kgm./hr. and the pressure release valve adjusted to maintain a pressureof 40 atm. in the mill. The steam was then turned on to the steam jacketof the mill, and the electric heaters for the water and the normallysolid polythene switched on. When the water, normally solid polyethyleneand mill temperatures had reached 150 C. the mill was started and run at7500 R. P. M. and the screw extrusion pump started so as to feed thenormally solid polythene mixture at a rate of 10 kgm./hr. The exit fromthe pressure release valve was at the bottom of an open l-litre tankfitted with an overflow delivering into a receiver. 2 litres of aqueousphase at 35 C. were circulated through this tank and through a cooler.The dispersion emerged from the release valve, mixed with and graduallydisplaced the aqueous phase in the cooling system, so that after mins. adispersion containing 30% normally solid polyethylene was beingcollected from the overflow. The average particle size of the dispersednormally solid polyethylene was 10 microns Dispersions made according tothe process of this invention are of most value in coating paper inorder to waterproof it, and in impregnating fabrics to stiffen them. Thenormally solid polythene retains its well known toughness, which is suchan important factor in its use for coating and impregnating materials.

What I claim is:

1. A process for the manufacture of polyethylene dispersions whichcomprise subjecting a mixture of molten, normally solid polyethylene andone to five parts by weight of water per part of polyethylene at atemperature above 115 C. and at a suflicient pressure to keep the waterin the liquid state, to a high rate of shear such that the dispersedparticles of said polyethylene have an average particle size of fromabout 5 to 20 microns, and then releasing the pressure.

2. A process as claimed in claim 1 carried out at a temperature between130 and 250 C. and at a pressure between 20 and atmospheres.

3. A process as claimed in claim 1 wherein between 2 and 5 parts byweight of water are used per part of normally solid polyethylene.

4. A process as claimed in claim 1 carried out in the presence of anemulsifying agent.

5. A process as claimed in claim 1 carried out in the presence of a soapas an emulsifying agent, the soap being made in situ by having a higherfatty acid dissolved in the normally solid polyethylene and a basedissolved in the aqueous phase.

6. A process as claimed in claim 1 in which an emulsifying agent isemployed and in which the amount of emulsifying agent is between 1 and10 parts by weight per parts of normally solid polyethylene.

'7. A process as claimed in claim 1 carried out in the presence of anemulsifying agent and at a temperature between and C.

8. A process as claimed in claim 1 in which the emulsion is rapidlycooled immediately after the pressure has been released.

9. A process as claimed in claim 1 in which the pressure is releasedwithin 10 seconds after J the dispersion has been subjected to a highrate References Cited in the file of this patent UNITED STATES PATENTSNumber Name Date 2,290,794 Alvarado July 21, 1942 2,313,144 Gomm Mar. 9,1943 2,386,674 Flint et al. Oct. 9, 1945 OTHER REFERENCES Carbide andCarbon Chemicals Corp.: Emulsions, seventh edition, April 15, 1946,pages 8 and 9.

1. A PROCESS FOR THE MANUFACTURE OF POLYETHYLENE DISPERSIONS WHICHCOMPRISE SUBJECTING A MIXTURE OF MOLTEN, NORMALLY SOLID POLYETHYLENE ANDONE TO FIVE PARTS BY WEIGHT OF WATER PER PART OF POLYETHYLENE AT ATEMPERATURE ABOVE 115* C. AND AT A SUFFICIENT PRESSURE TO KEEP THE WATERIN THE LIQUID STATE, TO A HIGH RATE OF SHEAR SUCH THAT THE DISPERSEDPARTICLES OF SAID POLYETHYLENE HAVE AN AVERAGE PARTICLE SIZE OF FROMABOUT 5 TO 20 MICRONS, AND THEN RELEASING THE PRESSURE.